This invention relates to an air conditioning system for automotive vehicles, which is equipped with a variable capacity compressor controllable by an external control signal.
Conventional air conditioning control systems for automotive vehicles include a type which is equipped with a variable capacity compressor which has its capacity controlled by an electrical signal supplied from the outside of the compressor, in other words, in which at least the suction pressure is automatically controlled to a constant value in response to an external control signal.
One of conventional air conditioning control systems of this type is known e.g. from Japanese provisional patent publication (Kokai) No. 58-43340, according to which the capacity of the compressor is controlled in response to the cooling degree of the evaporator and the ambient temperature encompassing the automotive vehicle.
Air conditioning control systems of this type, of which the capacity of the compressor is appropriately varied in response to conditions under which the system is placed, such as thermal load, are advantageous in consumption of the energy, as compared with one equipped with a fixed capacity compressor. However, they have difficulties in carrying out cooling-down control. Specifically, as shown in FIG. 9 for example, according to a system of this type in which the compressor capacity is controlled in response to the temperature within the vehicle compartment, the compressor capacity decreases as the compartment temperature lowers, resulting in poor cooling-down ability. Further, according to conventional systems of this type, the cooling degree of the evaporator is detected, and control of the compressor capacity is carried out so that the temperature of the evaporator does not lower below the freezing point. Therefore, the compressor capacity cannot be set to such a large value that the evaporator temperature lowers below the freezing point, in order to prevent the evaporator from being frozen.
As a result, according to conventional air conditioning control systems of this type, when the cooling degree of the evaporator is detected to be close to the freezing point, the compressor capacity is controlled to a reduced value so as to prevent freeze-up of the evaporator. Therefore, the variable capacity compressor is virtually inferior in cooling-down ability to fixed capacity compressors which can be operated with full capacity until the freezing point is reached.